How to reduce fireflies in Cycles?
I recall a few techniques for this. As mentioned by futurehack you can render twice with low samples using different seed values which puts the fireflies in different locations, when mixed together the fireflies get cancelled out.

An example of two renders with 10 samples and different seed values -


Turning up the filter glossy value (in light paths panel) can help.

Over at CGCookie you may find help with Sebastian's tutorial on reducing caustic noise. To summarize, for the glossy material you can use the 'Is Diffuse Ray' from a Light path node to choose between a glossy or diffuse material. This allows you to still get indirect lighting with 'No Caustics' turned on in the light paths while reducing noise.


I think the most helpful will probably be using the bilateral blur by Bartek. This tutorial gets a bit more in depth with compositing but essentially discusses different ways to define a mask that can be input into a bilateral blur node and which render passes can be blurred and combined for the best result. The determinator input of the bilateral blur node is used to define the edges of the area that gets blurred. This lets you limit the blur to specific areas that for example have fireflies.